1. Field of the Invention
This invention relates generally to an apparatus for recording and reproducing video and audio signals and, more particularly, is directed to such an apparatus suitable for application to a digital video tape recorder (DVTR).
2. Description of the Prior Art
Recently, the SMPTE "D-1 Format" has been adopted as an international standard for the digital recording of a conventional 4:2:2 component-type video signal in a digital video tape recorder. Reference is made to "Introduction to the 4:2:2 Digital Video Tape Recorder", published 1988 by Pentech Press Limited, London, for a detailed description of the conventional 4:2:2 digital video tape recorder, and more particularly for a detailed description of the tape format and the arrangements of the recording and reproducing heads used in such digital video tape recorder. However, for the purposes of understanding problems overcome by the present invention, it is sufficient to note that, as shown diagrammatically on FIG. 1, in accordance with the SMPTE D-1 format, digital video and audio signals are recorded on a magnetic tape T in successive parallel slant tracks A, B, C and D extending obliquely across the magnetic tape. Each such helical or slant track on the magnetic tape T is comprised of two video sectors indicated at V1 and V2 and an intervening area AU for the recording of digital audio data.
In view of the large amount of data that needs to be recorded for each field of a video signal, it is not practical to digitally record a field on a single, unsegmented helical or slant track. Accordingly, in accordance with the D-1 format, a mixture of field segmentation and channel distribution are employed for recording each field in a number of the slant tracks so as to maintain within reasonable limits the amount of data that needs to be recorded in each slant track and hence the length of the latter. For example, for digitally recording a video signal of the NTSC or 525/60-System according to the D-1 format or standard, only the last 250 lines of each field are recorded, and 10 helical or slant tracks are employed for recording the information of one field. The 250 lines of a field are first divided into five segments, each consisting of fifty lines, so that two of the ten tracks employed for recording a field of video signals are available for each segment. Further, in order to ensure minimum sensitivity of the recorded signal to tape defects, such as dropouts and the like, the incoming video signal is distributed amount four adjacent tracks. For making the foregoing possible, the previously mentioned video sectors V1 and V2 are formed within each helical or slant track, and each segment of the video signal data is recorded in two pairs of video sectors located in four adjacent helical tracks, for example in the upper adjacent video sectors V2 of the first pair of tracks A and B at the left-hand side of FIG. 1 and the lower adjacent video sectors V1 of the next pair of tracks C and D.
In the case of the digital recording of a component video signal of the PAL or 625/50-System according to the SMPTE D-1 format, the data for each field of the video signal are similarly subjected to field segmentation and distribution among four channels so that, as shown on FIG. 1, the data for each field of the video signal is recorded in the twelve successive slant tracks indicated in solid lines on the drawing.
For recording audio and video signals of the PAL system according to the D-1 format illustrated on FIG. 1, a conventional digital video tape recorder (DVTR) employs two pairs of rotary magnetic heads mounted at diametrically opposed locations on a tape guide drum, with the two magnetic heads of each pair being mounted close to each other in substantially side-by-side relation so as to simultaneously scan respective adjacent slant tracks on the tape. During recording, the tape T is advanced or transported at a predetermined standard speed in the direction of the arrow t, while the rotary magnetic heads scan the tape in the direction of the arrow h. The magnetic tape T is wound around a portion of the periphery of the tape guide drum which includes a tape-wrapping angle of approximately 260.degree.. Each pair of the rotary magnetic heads is supplied with video signal data for recording in the lower video sectors V1 of the adjacent slant tracks being scanned by the heads while traversing the initial 120.degree. of the tape-wrapping angle, whereupon audio signal data is supplied to the rotary magnetic heads during the scanning thereby of the next 20.degree. of the tape-wrapping angle, and finally video signal data is again supplied to the rotary magnetic heads during the scanning of the tape over the final 120.degree. of the tape-wrapping angle.
In the reproducing or playback mode of the standard DVTR, the digital video and audio signals thus recorded on the magnetic tape are reproduced by two pairs of rotary magnetic heads mounted on the tape guide drum at diametrically opposed locations, and which may be the same as the rotary magnetic heads used for recording. It will be appreciated that the rotary magnetic heads used for reproducing or playback of the video signal data recorded in the sectors V1 and V2 of the slant tracks respectively scanned thereby are also used for reproducing the audio signal data recorded in the respective audio areas AU which also have recorded therein time code signals for identifying the video signal data recorded in the respective tracks. During recording and normal playback or reproducing, one pair of the rotary magnetic heads located adjacent each other scans the slant tracks A and B simultaneously during the rotation of the tape guide drum through 180.degree., and the other pair of the rotary magnetic heads located adjacent to each other then scans the slant tracks C and D simultaneously during the turning of the tape guide drum through the next 180.degree..
In the case of the digital recording of video and audio signals of the PAL system, as indicated by the solid lines on FIG. 1, one field of the digital video signal is recorded during three revolutions of the tape guide drum starting from the upper video sectors V2 of the 1st and 2nd slant tracks A and B at the left-hand side of FIG. 1, and terminating at the lower video sectors V1 and the audio areas AU of the 13th and 14th slant tracks A and B, also counted from the left-hand side of FIG. 1.
With the above generally described standard DVTR, the recording and reproducing of high quality video and audio signals according to either the PAL system or the NTSC system are reliably achieved so long as the speed of tape transport used during playback is the same as the standard tape transport speed used during recording. However, if a variable tape speed is used during playback or reproducing, the quality of the resulting displayed picture may be deteriorated.
In the case of conventional analog VTRs, for example, of the type employing the C-format, if playback is effected with a tape transport speed different from that used for recording, each reproducing rotary magnetic head scans the magnetic tape T along a path intersecting a plurality of the slant tracks in which different fields of video information are recorded. By reason of the foregoing, when the reproduced signal is displayed on a monitor, several horizontal band-shaped noise patterns appear in the reproduced picture. In order to achieve variable tape speed playback without the generation of a noise component in the reproduced signal, it is known in the prior art to provide an analog video tape recorder, for example, as disclosed in U.S. Pat. No. 4,549,235, having a common assignee herewith, in which a reproducing rotary magnetic head is mounted on a head moving device, such as, a bimorph leaf, so that the reproducing rotary magnetic head can be displaced in the direction transverse to the slant track being scanned thereby. Thus, during playback with a tape transport speed different from that used in recording, a suitable head deflecting signal is applied to the bimorph leaf for ensuring that the respective reproducing rotary magnetic head accurately scans or tracks a slant track on the magnetic tape.
Referring now to FIG. 2, it will be seen that in applying the above teaching to a conventional 4:2:2 DVTR, a first pair of reproducing magnetic heads P(A) and P(B) and a second pair of reproducing magnetic heads P(C) and P(D) are mounted on bimorph leaves BM1 and BM2, respectively. The leaves BM1 and BM2 are mounted on a rotary drum DR in diametrically opposed directions so as to locate the respective reproducing magnetic heads at substantially equally angularly spaced positions relative to a first pair of recording heads R(A) and R(B) and a second pair of recording heads R(C) and R(D) which are fixedly mounted at diametrically opposed locations on the periphery of the drum DR rotated in the direction of the arrow r.
If the conventional 4:2:2 DVTR is used for recording a PAL television signal, one field of such signal is recorded in 12 tracks on the tape T, as shown in FIG. 3. Since each field period of a PAL television signal is 1/50 second, the rotary drum DR has to be rotated three times during 1/50 second so as to cause the recording heads R(A), R(B), R(C) and R(D) to scan 12 slant tracks on the tape for the recording of one field of the PAL television signal therein. In other words, a rotational speed of 150 r.p.s., or 9,000 r.p.m. is required. The track pitch in the tape format for the standardized 4:2:2 DVTR, for example, the transverse distance between the adjacent tracks A1 and B1 in FIG. 3, is 45 micrometers so that, for the case where one field of a PAL television signal is recorded in 12 tracks on the tape T, the overall pitch or distance along the tape covered by a set of 12 tracks in which one field is recorded is 540 micrometers. Further, in the case of the standardized 4:2:2 DVTR, the tape T is wrapped around a portion of the periphery of the rotary drum DR including an angle of 257.7.degree. so that, in the playback or reproducing mode, each of the bimorph leaves BM1 and BM2 is flexed or deflected while the respective playback heads move through an angular extent of 257.7.degree. and scan respective tracks on the tape, and each of the bimorph leaves BM1 and BM2 is returned to its neutral or unflexed condition while the respective playback or reproducing heads are out of contact with the tape, that is, are moving through the angular extent of 102.3.degree. where the tape is not wrapped about the rotary drum DR.
Considering initially the situation in the twice normal tape speed playback mode, that is, a playback operation in which the magnetic tape is transported at twice the normal tape speed used for recording, it will be appreciated that the playback head P(A) begins scanning a slant track at the point P on FIG. 3 and finishes scanning the first track A1 (counted from the left-hand side of FIG. 3), the fifth A2, the ninth track A1 and the eleventh track A2 at the point Q1. During such scanning by the head P(A), mistracking is avoided by suitable flexing or deflection of the bimorph leaf BM1. The required amount of such flexing of the bimorph leaf at its outer end carrying the respective playback heads is 45.times.12.times.257.7/360=387 micrometers. In the still playback mode of the standardized 4:2:2 DVTR, the bimorph leaf BM1 similarly needs to be displaced or deflected by 387 micrometers, but in the direction opposite to the flexing or displacement of the bimorph leaf for the twice normal tape speed playback mode. Thus, the bimorph leaf BM1 has to be capable of a maximum overall or peak-to-peak displacement of 774 micrometers, that is, a displacement of 387 micrometers from its neutral position in each of the opposed directions. Such overall maximum displacement of 774 micrometers can only be realized with substantial difficulty and, in any event, is the upper limit of the displacements that can be achieved with a bimorph leaf of a size that can be mounted on the rotary drum of the standardized 4:2:2 DVTR.
Further, after the signals recorded in the twelve slant tracks from the point P to the point Q1 on FIG. 3 have been reproduced in the twice normal tape speed playback mode, the next twelve successive slant tracks are skipped from the point Q2 on the slant track A1, and then scanning is commenced beginning at a point along the next track A1 which positionally corresponds to the point P on FIG. 3. Thus, the audio areas AU of the tracks indicated at e on FIG. 3 are not reproduced. As earlier noted, in accordance with the SMPTE D-1 format, a time code signal for indicating the tape position is recorded along with the digital audio signal in the audio area AU of each track. Therefore, when the audio areas AU indicated at e on FIG. 3 are jumped or not reproduced, the respective time code signal cannot be read. If the rotational speed of the rotary drum DR is increased so as to permit the time code signal in the audio track areas AU indicated at e to be read within one field period, the increase in rotational speed has to be about 20% so as to accommodate the format of the 4:2:2 component DVTR. Such 20% increase in the rotational speed of the drum DR serves to increase the rotational speed from 9,000 to 10,800 r.p.m., that is, to a speed of 175 r.p.s. When the rotary drum DR rotates at 9,000 r.p.m., the centrifugal force acting on each of the bimorph leaves BM1 and BM2 is about 3,000 G and, when the rotational speed is increased by 20%, the centrifugal force acting on each of the bimorph leaves is 3,000.times.1.2.sup.2 =4,200 G. It has been observed that, when such increased centrifugal force is applied to the bimorph leaf, the maximum possible displacement of the bimorph leaf is reduced from the previously mentioned 774 micrometers to 645 micrometers which is less then the overall displacement required for ensuring proper tracking in the twice normal tape speed mode and the still playback mode.
Further, in the 1.9.times.normal tape speed playback mode, that is, the reproducing mode in which the tape is transported at 1.9 times the speed used for recording, the trace along which a playback head scans across the tape is increasingly displaced during successive scans from the slant track that should be scanned by such head. For example, the playback head P(A), when starting to scan the first of the group of slant tracks in which one field of a digital video signal is recorded, may be displaced from its desired position by as much as 45.times.12=540 micrometers. Consequently, the maximum displacement or deflection of the bimorph leaf BM1 needed for maintaining tracking may be 322+540=862 micrometers, which substantially exceeds the maximum displacement that can be achieved with a bimorph leaf of the size that can be employed in a DVTR according to the D-1 format.
Instead of mounting the reproducing rotary magnetic heads on bimorph leaves which are flexed so as to achieve accurate tracking of the slant tracks in a DVTR during variable tape speed playback, that is, when the tape is transported at a speed different from that used for recording, it has been proposed to increase the number of the reproducing rotary magnetic heads fixedly mounted on the rotary drum, for example, by employing eight reproducing rotary magnetic heads in diametrically opposed groups of four, rather than the two pairs of diametrically opposed reproducing rotary magnetic heads previously provided on the rotary drum. In such case, and assuming that each reproducing rotary magnetic head can accurately reproduce a digital video signal recorded in the slant track being scanned thereby so long as the head is not displaced from such slant track by a distance greater than 1/2 the track pitch, noise-less variable tape speed playback can be achieved when the tape transport speed is within the range from -1 to +1 times the tape transport speed used for recording. In the case of variable tape speed playback with tape transport speeds outside the foregoing range, it is necessary to further increase the number of reproducing rotary magnetic heads if noise-less playback is to be achieved thereby. However, further increasing the number of reproducing rotary magnetic heads is disadvantageous in that it undesirably increases the size of the rotary magnetic head assembly.